With automatic transmissions for motor vehicles, as is known, for example, from DE 198 58 541 A1, the transmission stages are adjusted by frictional-locking shift elements, which are to be understood in the following as frictional-locking clutches or brakes. In the disclosed automatic transmission, clutches, which connect two rotatable elements to each other in a torque-proof manner, and brakes, which connect in a torque-proof manner a rotatable element with a fixed element or an element connected in a torque-proof manner to the transmission housing, are formed as multi-disk shift elements. In this case, the actuation of the shift elements takes place hydraulically; i.e., by means way of actuators in the form of pistons/cylinder units, which are subjected to pressurized oil. The pressurized oil is applied using a motor-driven pump as a pressure source and, throughout the entire operating period of the automatic transmission, must be maintained at a pressure level that produces a compression force between the multi-disks in the shift element that is sufficient for the transfer of torque. The energy used to generate pressure by means of the hydraulic pump must be applied by the engine of the motor vehicle, such that the generation of hydraulic power increases fuel consumption and CO2 emissions or, because of the power losses, reduces the energy available to use to drive the vehicle. Thus, the degree of efficiency of the transmission is reduced.
In principle, the shift elements of an automatic transmission may be formed, apart from frictional-locking multi-disk shift elements, as positive-locking shift elements, such as claw shift elements.
In addition, leakage losses arise at sealing points, such as pressurized oil supply lines from the transmission housing through so-called “rotary oil supply lines,” which are sealed by means of gap seals, such as slide bearings and/or rectangular rings, in the rotating transmission shaft. This requires an ongoing tracking of the oil pressure in the actuator with a locked shift element or a replenishment of the leakage amount, in order to keep the shift element locked.
In order to make the pressure in the actuator independent from the pressure of the pump and keep the leakage losses to a minimum, the piston chamber may be blocked through various locking mechanisms, such that the pressure remains in the piston chamber and no additional oil must be conveyed. Only during the shifting process is the valve opened and then filled with the corresponding pressure. A hydraulic control device is known from DE 102 05 411 A1 of the applicant; by means of this device, if a torque transfer is desired, the multi-disk shift elements closable through pressurization are locked outside of the shifts by means of a locking device. Thereby, the pressure in the actuating cylinder acting as an actuator, and thus the compression force between the multi-disks, is maintained without hydraulic pressure identical to the actuating pressure of the shift element having to be generated by the gear pump.
Thus, the pressure to be generated by the transmission pump can be reduced relative to the actuating pressure. Although theoretically possible, the pump is not completely switched off or pressureless, since, even with closed and locked shift elements, the need for oil at low pressure continues to exist in the transmission, for example, for cooling and lubrication, or for pre-filling the shift element in preparation for a shifting process.
In this manner, the power consumption of the transmission oil pump, which is calculated as the product of the conveyed volume flow and the generated pressure difference, is significantly reduced. With a lower power consumption of the transmission pump, the overall degree of efficiency of the transmission increases, since less engine power has to be diverted for the hydraulic system and is available to the vehicle drive. A locking device disclosed in this prior art is formed as a stop valve, which is arranged in the feed area of the hydraulic cylinder and tightly blocks the pressure chamber of the cylinder at the pump, such that the hydraulic pressure necessary for maintaining the contact pressure in the multi-disk pack is maintained.
The problem with the existing solution is that, for each shift element, a valve is required; this valve encloses the pressure in each piston chamber, hereinafter also referred to as an actuating pressure chamber. The disadvantage here is that, on one hand, the control pressure must be brought to several different points in the transmission; on the other hand, the possibility of error is significantly higher, since each individual valve can jam, etc. In this manner, the functional reliability of the automatic transmission is disadvantageously affected, since, in special cases, such as during engine standstill, the blocking of the automatic transmission may arise. Thus, the automatic transmission is also blocked against a tow movement. In order to, nevertheless, guarantee functional reliability or detect the position error, position sensors would be necessary for each shift element. In addition, the number of parts is increased by the large number of stop valves. This represents a high expenditure for assembly operations, installation space and costs.